The present invention relates to electronic devices and, more particularly, to electronic devices for monitoring the condition of an AC source and, in the event of unsatisfactory availability of AC power, for providing a signal indicating the unsatisfactory nature of the AC power. Either single-phase or multiple-phase power sources may be monitored.
As is well known, commercial AC power sources may experience periods during which the AC voltage slumps below its nominal value. In addition to periods of low voltage, commercial AC power sources may drop out, or fail to provide power, for periods ranging from one or two cycles of the AC power to extended periods of power outage.
Electronic circuits powered by AC electric power conventionally include a DC power supply for producing, from the available AC power, one or more sources of DC power usable by the electronic circuits. Such DC power supplies usually include filtering circuits for smoothing out the AC ripple which would otherwise be superimposed on the desired DC. Filtering circuits include energy-storage devices, usually including one or both of capacitors and inductors, capable of maintaining a satisfactory level of power for operation of the electronic circuits for a period of time following a power outage. Thus, rather than suddenly ceasing after an AC power outage, the output of a DC power supply decays at a rate dependent upon the capacity of the energy-storage devices contained therein and on the demands of the load. When AC power resumes, the need to supply energy to the energy-storage devices usually slows the rise in output of a DC power supply. The rate of decline and the rate of rise following loss and resumption of AC power, respectively, is usually unequal. That is, it may take more or less ti to recover a predetermined output of a DC power supply from a predetermined decay level than it does to reach the predetermined decay level from full output.
Many DC loads fed by a DC power supply may be severely affected by loss of DC power. For example, a load including a computer and peripherals may lose vital data from volatile storage devices if a DC failure occurs without steps being taken to avoid data loss. In addition, non-volatile storage devices may lose synchronism with clock and interrupt signals. Further, control-line outputs of the computer may become indeterminate causing corruptive system operation such as, for example, overwriting areas of random access memory. The recovery time from an unprotected power outage including, for example, reconstructing lost data and re-initializing gate and computer circuits may easily become unacceptable.
Some prior-art power-source monitoring devices detect the loss of AC power. Typically, such monitoring devices respond to the absence of a predetermined number of AC voltage cycles by producing a signal acted upon by the load to perform an orderly shutdown of the circuits using the energy stored in the energy-storage elements of the power supply. In this way, momentary dropouts in the AC supply of one or two cycles are ignored while permitting orderly shutdown of the system when the power outage persists for longer than the predetermined number of cycles.
A substantial motivation exists for keeping some electronic systems in operation, even in the presence of repeated, short-duration, AC power dropouts. The above prior-art device is limited to the detection of a single dropout of a predetermined duration. If power resumes before the predetermined duration is exceeded, and is followed closely by another dropout, the prior-art systems have no means for keeping track of the status of the energy storage in the DC power supply, except for direct measurement of the output voltage of the DC power supply.
Another indication of impending failure of AC power is found in a substantial decrease in the frequency of the AC power. The above prior-art power-monitoring devices do not take advantage of this indication.